We propose to examine the in vivo role of fibrinogen in hemostasis and thrombosis using a mouse model based on an inherited abnormal fibrinogen. This fibrinogen, called Vlissingen/Frankfurt IV (V/FIV), was found in two related patients, both heterozygous, who presented with severe thrombotic disease prior to age 40. Our own biochemical characterization of recombinant V/FIV fibrinogen demonstrated that this protein does not clot and does not support platelet aggregation. Thus, one would predict a bleeding phenotype, at least in homozygous individuals. After comparing recombinant fibrinogen to fibrinogen from patient plasma, we concluded that the heterozygous genotype is an essential aspect of the thrombotic phenotype and that the clinical disease arises from a dominant-negative mutation. With three specific aims in mind, we used gene-targeting methods to generate mice with the analogous abnormal fibrinogen. Preliminary experiments suggest that homozygous mice do, in fact, have a bleeding disease. The specific aims are: 1) to characterize the phenotype of the V/FIV mice and the biochemical properties of V/FIV fibnnogen isolated from these mice. These experiments will address two questions: Does the V/FIV fibrinogen genotype have pathologic consequences? Do the biochemical properties of mouse V/FIV fibrinogen correlate with the pathology? 2) to characterize the phenotype of heterozygous V/FIV mice in response to thrombotic challenge. These experiments will address the question: Does the heterozygous V/FIV genotype increase susceptibility to thrombosis? 3) to characterize the effect of atherosclerosis on the development of thrombosis in heterozygous V/FlV mice. These studies will answer the question: Does atherosclerosis induce or increase thrombosis in heterozygous V/FIV mice? We expect the results from these studies to increase our understanding of the role of fibrinogen in normal physiology and disease. We also expect to identify and/or further characterize environmental and genetic factors that modulate the penetrance and expressivity of symptomatic disease. Finally, we may also find that these mice are a useful model for studies to identify new treatment and diagnostic modalities.